This invention relates to a reproduction system for use in reproducing a sequence of image signals subjected to redundancy reduction coding.
Attempts have been directed to create a reproduction system comprising a read-only memory which is formed by a compact disk (referred to hereinafter as a CD-ROM). The reproduction system might be available for purposes of amusement and education. Such a CD-ROM is used as an image source in the reproduction system. For convenience of description, the CD-ROM may be called a main image recording medium.
In this event, a succession of images should be memorized in the form of a sequence of image signals in the CD-ROM along with a voice signal accompanying the images. Herein, it is to be noted that such a CD-ROM has a memory capacity which is in the order of 540 megabytes. On the other hand, a usual color image has an amount of information of 500 kilobytes at every frame and should be transmitted at a transmission rate of about 18 megabytes per second.
As a result, when such images are stored in the CD-ROM without any redundancy reduction, the image signals stored in the CD-ROM are reproduced only as, for example, a time period of thirty seconds. This makes practical use of the reproduction system difficult because a requirement in market is directed to a long-term reproduction time in the order of one hour.
In order to respond to such a requirement, an image must be inevitably subjected to redundancy reduction coding or efficient coding. Otherwise, the above-mentioned long-term reproduction can not be realized by the use of such a CD-ROM. More particularly, each image signal for a single frame should be compressed or reduced in redundancy into a compressed image signal of about 5 kilobytes in average. This means that the image signal should be compressed at a compression rate of 1/120.
Recently, it has been proposed that a standard audio signal recording CD-ROM be used as a color image recording CD-ROM which realizes reproduction time of as long as one hour. In this case, an image signal for each frame is recorded on the color image recording CD-ROM in the form of a predictive error signal appearing as a result of efficient coding carried out in a manner to be described later. At any rate, the predictive error signal is subjected to redundancy reduction or compression and may be referred to as a redundancy compression signal. Such efficient coding may be, for example, interframe coding. When the interframe coding is used to code an image including a still portion and a moving portion having a small area in comparison with the still portion, it is possible to reduce an amount of significant information at the still portion.
The above-mentioned reproduction system is very effective and convenient for continuously reproducing, from a CD-ROM, a sequence of predictive error signals. However, it is to be noted here that no consideration is made at all about reproducing a still image from the predictive error signal sequence which might be continuously read out of the CD-ROM. Such a reproduction of a still image will be called a still image reproduction hereinunder and is not simply carried out on reproducing the predictive error signal sequence.
In order to reproduce a still image from a sequence of predictive error signals, consideration might be made about a circuit arrangement which may be a combination of a reproduction system as mentioned above and an image memory connected to the reproduction system. Specifically, a sequence of reproduced or decoded image signals is reproduced by the reproduction system and is once successively stored in the image memory prior to a visual display on a display device, such as a cathode ray tube. With this structure, it is possible to visually reproduce not only a moving image but also a still image. The still image reproduction is possible by repeatedly reading the same reproduced image signal out of the image memory. However, the connection or arrangement of the image memory makes the circuit arrangement intricate in structure. Moreover, it takes a long time for the circuit arrangement to switch from a moving image reproduction to a still image reproduction because the reproduced image signals are visually displayed through the image memory.